"The Maturational Theory of child development was introduced in 1925^[1] by Dr. Arnold Gesell, an American educator, pediatrician and clinical psychologist whose studies focused on "the course, the pattern and the rate of maturational growth in normal and exceptional children"(Gesell 1928).^[2] Gesell carried out many observational studies during more than 50 years working at the Yale Clinic of Child Development, where he is credited as a founder. Gesell and his colleagues documented a set of behavioral norms that illustrate sequential & predictable patterns of growth and development. Gesell asserted that all children go through the same stages of development in the same sequence, although each child may move through these stages at their own rate ^[3] Gesell's Maturational Theory has influenced child-rearing and primary education methods since it was introduced.^[4][5] (Wikipedia article on Gesell's Maturational Theory, accessed 05-2018). Digital facsimile from the Internet Archive at this link. Various films made by Gesell and/or showing him at work are available on YouTube.

Gesell, who originated the Child Study Center at Yale University, was the founder of the study of child development in the United States. He is best known for his groundbreaking studies of normal child development: beginning in the 1920s, he used advanced cinematic and photographic techniques, including one-way mirrors, to record developmental milestones from infancy to adolescence. His most famous work is the Atlas of Infant Behavior, which contains 3200 photographs documenting the human infant's "visible manifestations of his maturing patterns of action and reaction. . . . Through systematic, pictorial charting, we trust that this Atlas will reveal the patterned organization of the moments and of the developmental sequences of infant behavior" (p. 11).